1. Field of the Invention
The present invention relates generally to the field of torque-angle wrenches and, more particularly, to a torque-angle wrench including a piezoelectric gyroscopic sensor to measure the tightening angle.
2. Description of the Prior Art
The object of wrenching tools is to rotate or hold against rotation an item, such as a threaded fastener joining two objects together. There is a relationship between the amount of torque that is applied to the head of a fastener and the amount of load applied to the joined objects. A torque wrench takes advantage of this relationship by measuring the torque applied as an indication of the joining force or load.
Torque is considerably influenced by friction forces, the condition of the head, the amount, if any, of lubrication, as well as by other factors. Accordingly, the reliability of a torque measurement as an indication of desired load is significantly variable. For this reason, a torque-angle fastener installation process, rather than torque measurement alone, is recommended in situations where tightening to recommended specifications is critical.
In a torque-angle fastener installation, a fastener is first tightened to a desired torque using a torque wrench; then the fastener is rotated through a predetermined additional angle of rotation. It is well understood in the industry, that the amount of load that a fastener applies in squeezing two objects together is more closely related to stretch or elongation of the fastener than it is to the torque applied, since friction forces, lubrication, and other factors have considerably less influence on the stretch of the thread as measured by the angle of rotation of the thread with a known pitch than they do on the torque applied. Because angle-based torquing is a more accurate way to ensure even tightening, more and more manufacturers are using the torque-angle procedure for tightening fasteners. Another advantage of torque-angle installation is that like fasteners exert the same clamp forces without deviation from one fastener to the next because of variable conditions of lubrication, surface finish and the like.
At present, there are various wrenching tools available which meter angular rotation. Early angle measurement wrenching tools relied on some type of mechanical reference, usually a flexible strap connected to a "ground" clamp, for measurement of the angular rotation of a fastener.
More modern tools now use gyroscopes to meter angular rotation. One such device is disclosed in U.S. Pat. No. 4,262,528 to Holting et al. A gyroscope operates by offering opposition to a swiveling motion around an axis located transversely to its axis of rotation. The Holting gyroscopic wrench includes a gyroscope rigidly connected to a blade element interposed between a set of coils. The gyroscope has a rotor which defines the spin axis of the gyroscope. The gyroscope is mounted onto the tool via a support member in a manner which permits directional changes of the spin axis orientation from an initial orientation, due to precession of the rotor during rotation of the tool through the tightening angle. An electrical signal representative of the magnitude of rotor precession is generated by a sensor. The signal is then fed to a device which operates to return the gyroscope to its starting (neutral) position. The current intensity of the signal is proportional to the gyroscopic motion which occurs at the gyroscope support member, at a predetermined angular velocity around the pivoting axis. Accordingly, the signal, integrated by an appropriate integration circuit, is proportional to the tightening angle of the wrench about the axis of fastener rotation. The integrated signal thus provides a visual indication of the angle of wrench rotation.
Gyroscopic devices have gained in popularity over the years despite their non-negligible power consumption and the bulkiness of their respective housing units, in each of which is mounted a spinning gyroscope, a rotor, as well as appropriate integration and signal amplifying circuitry. The fact that gyroscopic units do not require a flexible `ground` or `reference` strap also is believed to have contributed to their popularity. However, high power consumption, a bulky construction, high manufacturing costs, and the need for greater accuracy has many scientists and engineers striving to come up with a better, more efficient torque-angle wrench.
The use of piezoelectric elements to perform torque measurements is well known. However, piezoelectric gyroscopic elements have never been used to measure `rotation` of a fastener during a torquing operation.